1. Field of the Invention
The present invention concerns a device for rotational moulding for the production of an object, manufactured of a material containing a curable raw material, wherein the device comprises a mould which is movably arranged, having a mould wall, surrounded by a mould cavity and a material feeding device for feeding the curable raw material into the mould cavity, according to the first claim.
2. Description of the Related Art
In the production of large numbers of the same form of plastic, often use is made of techniques such as blow moulding, and injection moulding by extrusion of a plastic in a mould. Another much used technique for the manufacture of the skin of tension-free hollow objects in smaller numbers is rotational moulding. Rotational moulding offers the possibility to produce hollow shapes, the skin of which consists of one or more layers that are build up of different materials, for example a lower layer in plastic and a upper layer in a finishing material, for example pieces of glass, or objects of which the skin thickness should be locally different. In known techniques for rotational moulding of plastic, a mould is filled with an amount of plastic, after which the mould is placed in an oven for heating and melting of the plastic. Once the plastic has melted, the mould will be rotated and optionally agitated to obtain the desired distribution of the molten plastic in the mould. Subsequently, the mould is cooled and the object is demoulded. A skin that contains two or more layers is often produced using a hopper or dropbox connected mould, the hopper containing the raw material for the production of the second layer. Although mould and dropbox are thermally insulated, the connection between the two may give rise to a large number of rejected products. Rotational moulding can be a cumbersome technique when working with multiple moulds and dropboxes. Other disadvantages of the technique of rotational moulding are a high energy consumption when using indirectly heated moulds wherein the moulds, for example, are heated by placing them in an oven, unsafe conditions, when directly heating with a flame and limited automation and process control capabilities.
WO2006/103179 in the name of Krauss Maffei discloses a device for slush-moulding or pour moulding, as well as a method for the manufacturing of a plastic skin for a hollow object. The device contains a mould for shaping the plastic skin, mounted on a centrally arranged robot arm. According to a first embodiment, the mould can be attached to a container with a plastic material stock. According to an alternative embodiment, the plastic container and the mould are mounted on separate robot arms and they are coupled to allow dosage of plastic to the mould. A heating device is arranged in such a way that it can receive the empty mould for the indirect heating of the mould. The use of an indirect heating of the mould requires that the mould as well as the peripheral (hydraulic) components are temperature resistant. The robot arm is provided for the displacement of the mould to the inner space of the oven for heating of the mould, and from there to the plastic material feeding device for forming a link for the feeding of plastic from the plastic material feeding device to the mould. The link with the heated mould brings along a risk of clogging of the compound in case of poor insulation. The filled heated mould is subjected together with the plastic material feeding device to rotation, shaking and pivoting movement to form the object. Thereafter, the mould and the plastic material feeding device are separated and led to a heating oven for the gelling of the plastic. This heating of the mould and plastic in two steps is time consuming and disadvantageous for the quality of the final product. After the second heating step, the mould is moved to a cooling device and from there to a removal station for the removal of the plastic object from the mould. It is possible to disconnect the mould from the robot during the gelling and cooling step, allowing the robot to be accessible for the manipulation of other moulds. Plastic material feeding device, oven, cooling device and removal station are mounted on a circular orbit around the robot. Further disadvantages of these known devices are that the mould has to be moved from one station to the other in order to run through all the cycle steps of filling, heating, cooling and removing the plastic skin. If used with multiple moulds, the total cycle time for running through all steps is therefore determined by the cycle time of the slowest mould.
A similar carrousel system with different stations but then for rotational moulding is, for example, also disclosed in XP002681519.
US2007/0063370A1 discloses a device for rotational moulding that aims to replace such carrousel systems with different stations. However, US2007/0063370A1 also has the disadvantage that the mould cannot yet be sufficiently simultaneously subjected to a temperature-time program during a plurality of operations, movements, and a displacement course. For the feeding of curable raw material to the mould, the temperature-time program still has to be interrupted to perform an additional operation with the aid of a robot arm to a feeding device. This has as a consequence that, if several material layers are desired in the object to be produced, still an insufficient control over the temperature-time program is obtained.
EP1.649.997 in the name of Persico SpA discloses a device for rotational moulding, wherein the wall of the mould comprises integrated flow channels for a hot or cold liquid, respectively, for heating and cooling of the mould.
Consequently, there is a need for a device that is suitable for rotational moulding, that offers the possibility of producing an object, that may contain a plurality of material layers that are superimposed upon each other in such a way that a final product can be provided that is build up of a raw material combination that is difficult to achieve with existing techniques. Furthermore, there exists a need for a device for producing large amounts of the same object with an increased production capacity.